The use of solar energy has received a great deal of attention, particularly in connection with space heating. For this purpose the "flat plate" solar collector has been the preferred type. In the typical embodiment, this comprises a shallow box, covered with glass or other transparent material. Within the box are pipes carrying water, usually with an antifreeze additive. Frequently the pipes are attached to metal plates to increase the absorbent surface. The bottom and sides of the box are insulated. The water is used to supply heat directly to the space to be heated.
Heat pumps are also well known for space heating. Typically, they involve a condenser within the building to be heated and an evaporator exposed to the ambient air, though there have been systems in which the evaporator absorbed heat from ground water.
In many areas of the United States, solar heaters have not been an attractive source of residential energy due to the high capital cost of the flat plate collectors required to supply a significant amount of heat during the winter.
Attempts have been made to combine the solar heater with a heat pump to enhance the efficiencies of both. These have been centered on the use of conventional flat plate collectors coupled, by means of a heat exchanger, to the evaporator of the heat pump. This permits the collection system of the solar collector to operate at a lower temperature and the evaporator of the heat pump to operate at a higher temperature than would otherwise be the case. Although efficiencies did rise with this type of system, the installed cost of the solar collectors prevented significant increases in return of investment from being achieved.
A variation of the systems just mentioned is disclosed in U.S. Pat. No. 2,529,154 to E. H. Hammond et al. In the system disclosed by the patent, water or an aqueous solution is circulated through a solar collector mounted in a rooftop and serves to heat the earth or a pool of water. A heat pump system absorbs heat from the water or earth and delivers it to a house.
U.S. Pat. No. 1,765,136 to C. H. Drane, Jr., shows a steam generator involving a solar collector and a heat pump. Ammonia is evaporated in a solar collector. The vapor is compressed and is condensed in a heating coil located in a steam boiler. The solar collector is of the conventional, flat plate, insulated box type operating generally above ambient temperature.
U.S. Pat. No. 3,262,493 to D. E. Hervey shows a system including a solar heat exchanger "adapted to selectively, in the alternate, collect heat and dissipate cold or collect cold and dissipate heat". It consists of a series of flattened tubes each mounted in a semi-cylindrical reflector so that the tubes receive both direct and reflected sunlight. The reflectors appear to be of the specular type. The tubes appear to be exposed to the atmosphere. No heat pump is included. Heat absorbed in the collector by circulating fluid is stored in, and later extracted from, the soil.
U.S. Pat. No. 3,991,938 to Harry Borders Ramey shows a system in which a plate on the ridge of house is sprayed with water and is cooled by the evaporator of a heat pump. The temperature of the plate is kept low enough to freeze the water, thus extracting heat. The heat pump is reversed at intervals, allowing the ice to slide down under a transparent cover, where it is melted and warmed by solar heat. The warmed water is first used to warm the house, then to cool the condenser of the heat pump, absorbing heat, again used to heat the house, then returned to the ridge plate.
U.S. Pat. No. 4,002,160 to George R. Mather shows a solar heat collector including a diffuse reflecting surface and a series of blackened collector tubes spaced from the reflecting surface and from each other. Each blackened tube is surrounded by a clear tube and the space betwen them is evacuated, so that the blackened tubes are insulated from the surrounding air. No heat pump is utilized.